Method and apparatus for growing single crystals



March 20, 1962 cHlH-cl-IUNG WANG 3,026,188

METHOD AND APPARATUS FOR GROWING SINGLE CRYSTALS Filed, April ll, 1960 FIG.|

JNVENTOR cHlH-CHUNG wANG ATTORNEY United States arent i 3,926,188 METHOD AND APPARATUS FR GROWING SINGLE CRYSTALS Chih-Chung Wang, Lexington, Mass., assigner to Clevite Corporation, Cleveland, Ghia, a corporation of Ohio Filed Apr. 11, 1960, Ser. No. 21,195 11 Claims. (Cl. 23-301) This invention relates to methods and apparatus for growing single crystals, particularly single crystals of semiconductor materials such as germanium and silicon.

Two principal methods are currently employed for growing single crystals, viz., the Czochralski method and zone leveling. `In the Czochralski method and variations thereof (e.g., the Stockbarger method) the crystal is pulled from the melt. Basically this method involves using a material to be crystallized, bringing a small single crystal seed into contact with the surface of the melt, and slowly withdrawing the seed so as to allow the melt to freeze progressively onto the seed forming a single crystal or" bar-like configuration. Depending on the material being crystallized and its end use, the method may be carried on in controlled atmospheres. The effect of the rate of pulling, melt temperatures, and other process parameters on the crystal produced are known and subject to adjustment to suit particular needs. The basic technique also is varied, as by rotating the crystal relative to the melt, establishing a vertical temperature gradient across the freezing zone, etc.

In the manufacture of semiconductor devices it is important to use semiconductor crystals which have a resistivity falling within a given range, as this affects the variation of electrical characteristics in the completed articles.

Unfortunately, crystals grown from the melt by the pulling methods alluded to above are characterized by a resistivity gradient caused by variation in the concentration of impurities incorporated into the crystal as will now be explained.

in growing crystals for semiconductors the melt of highly purified semiconductor materials, e.g., germanium or silicon, is doped with donor or acceptor impurities which give the crystallized material the desired type of conductivity and affect other important properties, including resistivity. The concentration of the impurity incorporated. in the crystal is a function of the segregation constant; the physical laws governing segregation are wellknown and well-understood. However, inasmuch as the volume of the melt diminishes, as the crystal pulling proceeds, the impurity concentration in the melt and, therefore, that in the crystal varies continuously. Consequently when crystal growth is completed, a large fraction of its length falls outside the limits of the permissible range of variation in the doping concentration and must be reprocessed.

Comparatively recently, the now well-known zone leveling process was developed which, by maintaining a substantially constant volume of melt (i.e., molten zone volume), increases the yield of crystal within the permissible range of variation. Zone leveling, however, also has its disadvantages: (l) the crystal must be contained within and freezes in a boat which exerts restraint on the crystal; (2) the restraint exerted is particularly undesirable because it is asymmetrical; (3) the lack of complete radial symmetry complicates the problem of controlling thermal gradient in the newly grown crystal as required to secure a high degree of crystal perfection; and (4) there are resistivity variations transverse to the longitudinal (horizontal) axis of the crystal due to the directional convection currents in the molten zone near the crystal growing surface.

3,026,188 Patented Mar. 20, 1962 Some of the foregoing disadvantages may be avoided by the use of the more recently developed oating zone process. However, the floating zone process is not suited to the growth of germanium crystals and, Worse, places a sever limitation on the size of crystals grown.

Moreover, all of these basic types of crystal growing techniques are batch processes, and, consequently, are possessed of the usual ineiiiciencies inherent in such processes.

The present invention contemplates a method and apparatus for growing single crystals which avoid the disadvantages of both the Czochralski-type and zone-leveling-type processes. The method contemplated comprises preparing a bar of the material to be crystallized; yieldably supporting the bar Within a mold; and, while so supported, establishing a molten zone at one end of said bar and moving the zone longitudinally therealong.

The apparatus contemplated by the invention comprises an elongated hollow mold of refractory material having an internal configuration conforming to that of the single crystal to be grown and adapted to receive a polycrystalline body of the substance to be crystallized. A resilient, refractory liner is provided, adapted to be disposed within the form, and fabricated of a material not wet by nor reactive with the substance to be crystallized in its molten state. Means are provided for establishing a molten zone within the mold and longitudinally movable therealong.

The fundamental object of the present invention is to provide a novel method and apparatus for growing single crystals which avoid at least one of the disadvantages common to the prior art methods and apparatus outlined above.

More particularly, it is an object of the present invention to provide an improved method and apparatus for growing single crystals characterized by a high degree of perfection and uniformity.

Another object is the provision of a novel method and apparatus for growing doped single crystals of semiconductor materials which have substantially uniform resistivity throughout.

A further object is the provision of a novel continuous process for growing single crystals and apparatus for carrying out such a process.

Still another object is the provision of a method and apparatus for growing elongated single crystals of predetermined cross-sectional dimensions and configurations.

These and additional objects of the present invention and the manner of their accomplishment will be apparent to those conversant with the art from the following description and subjoined claims taken in conjunction with the annexed drawing in which,

FIGURE l is a fragmentary view, partly in section along the vertical axis, of apparatus in accordance with the invention; and

FlGURE 2 is a fragmentary view similar to FIGURE l schematically illustrating a modified embodiment of the invention adapted for growing single crystals by a continuous process.

The initial step of the method of the invention involves the preparation of a polycrystalline bar of the material to be crystallized. Where the crystal is to be used for semiconductor devices, the material must be of controlled purity. Assuming for the purposes of example and the ensuing description that the material to be crystallized is intended for semiconductor devices, significant doping impurities are incorporated therein at some suitable stage of preparation.

The polycrystalline ingot is machined, cast, or otherwise formed into an elongated body of symmetrical crosssection, preferably circular.

To facilitate operation as -a continuous process, a

number of such bars would be prepared in advance and may have means permitting them to be coupled together end-to-end.

At this stage, then, the material to be crystallized is in the form of an elongated polycrystalline body, hereinafter referred to as the charge bar. The charge bar is now ready for use in the growing of (i.e., conversion to) la single crystal. To this end the bar is enclosed in a suitable form or mold adapted to laterally envelop at least a substantial fraction of its length. The bar is yieldably supported within the mold, as by virtue of having the internal dimensions of the mold slightly larger than the external dimensions of the body `and disposing inthe resultant clearance space a suitable refractory resilient material hereinafter described in greater detail in conjunction with the description of apparatus for carrying out the method.

The orientation of the charge bar in space is not a controlling factor but it is preferred that it be disposed with its longitudinal axis substantially Vertical. In this position the contact pressure between the bar and the material yieldably supporting it in the mold is uniform about the entire contact area whereas if the position is horizontal or inclined the weight of the bar results in a higher contact pressure on its underside.

With the polycrystalline charge `bar mounted in the mold as described, a small single crystal seed body is placed in contact with one end of the b-ar. A molten zone then is established by any suitable heating means at the end adjacent the seed and slowly moved longitudinally along the bar. The trailing edge of the molten zone in contact with the monocrystalline seed freezes in single crystal form -as the zone progresses along the bar. The thermal expansion and contraction involved in the progress of the molten zone along the bar would, in the absence of the yieldable support, subject the solidifying region of the crystal to such stresses as to cause the formation of undesirable nuclei and interrupt the growth of a single crystal lattice. -In laddition to avoiding stresses due to thermal expansion, the method has the additional advantage of providing symmetrical heat distribution thereby further avoiding thermal stresses and eliminating impurity concentration gradients due to temperature differentials in the molten zone.

Apparatus for carrying out the principal steps of the method will now be described with continued reference to the drawings wherein like parts are designated by like reference numerals throughout the several views. The 'apparatus in its simplest form is schematically illustrated in FIGURE 1 wherein it is designated as a whole by Yreference numeral 10.

j The apparatus comprises a form or mold 12 having Y an internal cross-section conforming to that of the polycrystalline charge bar and the desired cross-section of the Y suitable refractory material may be employed. The inner diameter of mold 12 is suciently greater than the outer diameter of the charge bar, designated by reference number 14, that the mold is adapted to receive the bar coaxially therein with la preselected annular clearance space 16 occupied in service by a yieldable :liner 18.

Liner 18 may be a tubular sleeveof ya relatively pliable 0r yieldable material which is Vnot Wet by nor reactive with the molten phase of the material being crystallized at operating temperatures. For example, liner 18 may be a `sleeve of inorganic paper or woven fabric of quartz or ceramic fibers (known as Fiberfrax). Papers of quartz and Fiberfrax are commercially available. If necessary to obtain the desired degree of yield, liner 18 may include a layer of loosely matted fibers, such as glass, quartz or Fiberfrax wool, adjacent the inner surfaces of mold 12. Liner 1S may also be constructed of suitable material in the form of tape having a width equal to the circumference of the charge bar. Tape commercially available under the proprietary designation Refrasil is satisfactory for this purpose. The tape is wrapped around the bar, so that the longitudinal edges of the tape form a butt joint running along one side of the bar. Preferably, a second tape is wrapped over the first inv like manner but with the butt joint angularly displaced from, i.e., not in registration with, the joint of the first tape. One or both tapes may `also be spirally wound around the bar in which case the width of the tape would not need to be equal to the bar circumference.

Polycrystalline charge bar 14 may be loaded into the apparatus by inserting it rst into Vliner 18 and then slipping both liner and feed bar into mold 12. Alternatively the mold may be made in hemi-cylindrical sections clamped together about the charge bar and liner.

With the charge bar loaded in the mold, a single crystal seed (not shown) is placed in contact with one end of the charge bar. The apparatus includes suitable means for establishing -a molten zone in the charge bar and moving the molten zone longitudinally along the bar at a slow predetermined rate.

In the illustrated embodiment the heater means takes the form of a radio frequency induction coil 20 concentrically surrounding a relatively short section of mold 12. It will be understood, however, that a resistance heater or `any other suitable means may be employed for establishing the molten zone. A suitable drive mechanism, not shown, is provided for causing relative longitudinal movement between the heating element and the mold. This mechanism may be operative to move the coil with respect to a ixedly mounted mold or, conversely, the heating element may be stationary and means provided for axial translation of the mold.

In operation the apparatus is loaded as already described and the heating unit energized and positioned to establish a molten zone in the polycrystalline material adjacent the end in contact with the single crystal seed. When this has been accomplished the mold or heater, as the case may be, is moved longitudinally with respect to the other so as to cause the molten zone to travel slowly along the polycrystalline charge bar. At the trailing edge of the molten zone the material freezes in monocrystalline form, the stresses which otherwise would cause the formation of a polycrystalline ingot being relieved by the yieldable nature of liner 16.

A modified form of apparatus 10' in accordance with the invention enabling the growth of a single crystal of indeterminate length as a continuous operation is illustrated in FIGURE 2. This form of apparatus consists of a iixedly mounted tubular guide member 22 of a suitable refractory material such as graphite, quartz, or high melting point metal. Concentrically surrounding the middle section'of guide member 22 are several coils 20 of a radio frequency inductive heating system. As in the case of the previously described embodiment other specific means may be used capable of establishing a molten zone in material passed through the guide means.

Guide member 22 is adapted by virtue of its internal dimension and configuration to receive coaxially a hollow cylindrical mold 12 made up of a number of hemicylindrical sections 12a, 12b, 12C =12g.butted endto-end and longitudinally staggered onl opposite sides of a diametral plane through the axis of guide member 22. Mold sections 12a 12g may be half round sections of quartz tubing or other suitable material as explained in connection with the first described embodiment.

Each of the mold sections individually is substantially longer than the guide member 22 so that when a section is moved longitudinally through the guide member a substantial part of the mold section projects from at least one end of the guide member at all times.

At each end of guide member 22 are drive rollers or equivalent means adapted to move the mold sections through the guide member; in the illustrated embodiment, the drive rollers are arranged in pairs 24 and 26, respectively at the entry and exit ends of the guide member. 'Ihe circumferential surface of each of the individual drive rollers preferably is shaped to conform to the outer surface of the mold sections and is constructed of resilient material adapted to frictionally engage the mold sections. The speed of the drive rollers is synchronized. It will be appreciated that, if desired, three, four or more rollers may be provided at each end of guide member circumferentially spaced about its axis.

A liner 1S', in all respects identical to that already described but of indeterminate length, is provided. A polycrystalline charge bar is formed in the manner already described for bar 14, FIGURE 1, but in a number of sections having respective complementary end surfaces adapting them to be either simply butted together or physically intertted or coupled, as desired.

In operation the initial charge bar, with a seed crystal at one end, is inserted into a section of liner 18', and two or three of the mold sections 12a 12g clamped about the bar and sleeve so as to enclose a substantial portion of the respective lengths. This assembly then is slipped, seeded end iirst, between the drive rolls 24 at the entry of the guide member 22. Heating element 20 then is energized and the rolls set in operation so that the mold sections with the liner and charge enclosed therein are moved longitudinally into guide member 22. With progressive upward movement, as viewed in the drawings, the leading edge of the charge bar moves into the region enveloped by induction coils 20 and, in due course, fuses to form a molten zone which moves progressively along the charge as it traverses the guide means. Upon leaving the heated zone the leading edge of the charge bar freezes in monocrystalline form.

With continued upward movement the leading edges of mold 12 and the single crystal emerge from the exit end of guide member 22 and thereupon are engaged by exit drive rollers 26. Thus, a driving force is maintained on the elements even after their trailing edges move clear to entry drive rollers 24. Before this occurs a second section of charge bar, and liner and an additional hemi-cylindrical mold section, assembled in the same manner as already described, are fed into the apapratus. These additional elements pass under the entry rollers 24 and are drawn into the guide member and passed through the hot zone as previously explained. The butted ends of the charge bar sections are of course fused together when traversing the molten zone so that the single crystal emerges as a continuous bar. The single crystal bar may be cut off in desired lengths without interrupting operation of the apparatus. Thus, it will be appreciated that by continually adding charge bar section and mold and liner segments, any number of polycrystalline charge bars may be converted into a single crystal in a continuous operation.

To avoid discontinuities, liner 18', whether in the form of tape or a tubular sleeve, may be several times the length of the individual charge bar sections, which are slipped into the liner and in abutment with the respective preceeding bar sections.

While in both of the exemplary embodiments described herein yieldable liners 18 and 18 have been represented as separate elements, it is to be understood that, if desired, the lining material can be permanently or semipermanently applied to the internal surfaces of mold 12 or 12. This may be of a particular advantage in the apparatus shown in FIGURE 2 inasmuch as it would be a simple matter to apply the liner to the half-round sections of the mold and it would facilitate the feeding operation involved in use of the apparatus.

While there have been described what at present are believed to be the preferred embodiments of this inven- 6 tion, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, to cover in the appended claims all such changes and modifications as fall Within the true spirit and scope of the invention.

What is claimed and desired to be sucured by United States Letters Patent is:

1. Apparatus for growth of a single crystal comprising: an elongated hollow mold of refractory material having an internal configuration conforming with that of the desired single crystal to be grown and adapted to receive a solid polycrystalline charge body of the substance to be crystallized; a refractory liner of pliable sheet material, not Wet by nor reactive with said substance in its molten state, disposed Within and conforming to the internal surface of Said mold; and means for establishing a heated zone within said mold, longitudinally movable therealong.

2. Apparatus according to claim l wherein said liner includes a layer of loosely matted fibers adjacent the inner surface of said mold.

3. Apparatus according to claim 2 wherein said liner comprises a sleeve of inorganic paper.

4. Apparatus according to claim 2 wherein said liner comprises a tubular sleeve of a fabric woven of a refractory mineral ber.

5. Apparatus for growth of an elongated single crystal comprising: an elongated tubular mold made up of an assembly of mating hemi-cylindrical sections of refrectory material, said mold having an internal diameter slightly in excess of the single crystal to be grown and adapted to receive coaxially therein a cylindrical polycrystalline charge bar of the substance to be crystallized; guide means for slidably coaXially receiving said mold and maintaining said sections in assembled relation to form a hollow cylindrical mold; a pliable resilient tubular liner of refractory sheet material, not Wet by nor reactive with said substance in its molten condition, adapted to be coaxially disposed within said mold; means for establishing a heated zone within said guide means; and means for moving said mold longitudinally through said guide means.

6. Apparatus according to claim 5 wherein said tubular liner is made up of at least one flat strip of said material wrapped about the bar with its longitudinal edges in abutment.

7. Apparatus according to claim 5 wherein said tubular liner is a separate sleeve of matter inorganic fibers.

8. Apparatus for growth of an elongated single crystal according to claim 5 wherein said tubular liner is made up of a number of individual sections applied to the respective inner surfaces of said hemi-cylindrical sections of the mold.

9. Apparatus for growth of an elongated single crystal comprising a plurality of hemi-cylindrical substantially identical mold sections adapted for selective progressive assembly to form a hollow cylindrical mold; tubular guide means substantially shorter in length than each of said mold sections individually and adapted to coaxially receive and maintain said sections in assembled relation; drive means adjacent one end of said tubular guide means adapted to move said mold sections in assembled relation coaxially into said guide means; additional drive means acljacent the other end of said guide means, adapted to withdraw and maintain in assembled relation mold sections issuing from said guide means; pliable resilient liner means of refractory sheet material, not wet by nor reactive with said substance in molten condition, coaXially disposed Within said mold sections at least during their traverse of said guide means; and means fixed with respect to and surrounding a mid-portion of said guide means for establishing a heated zone therein.

10. A method of converting a polycrystalline bar of material to a single crystal including the steps of supporting said bar in contact with a single crystal seed and traversing a molten zone along the length of said bar from 7 8 an initial point adjacent said seed while maintaining said References Cited in the le of this patent bar Wrapped in pliable sheet material not Wet by nor re- UNITED STATES PATENTS active with the material of said bar in its molten phase.

1l. A method for growing single crystals comprising: lmepllamp "OMM-63 preparing a polycrystalline bar of the material to be crys- 5 Ome 150m ct' tallized; wrapping said bar in pliable sheet material not Wet by nor reactive with the material of `said bar in its FOREIGN PATENTS molten phaseand supporting said bar so Wrapped Within 797,205 Great Bmam June 25, 1958 a mold; placing a single crystal seed adjacent one end of said bar; while so supported, establishing a molten zone 10 OTHER REFERENCES at said one end of the bar; and traversing the zone longi- Pfann: Zone Melting (1958), pages 62 and 63.

tudnally along the bar. 

11. A METHOD FOR GROWING SINGLE CRYSTALS COMPRISING: PREPARING A POLYCRYSTALLINE BAR OF THE MATERIAL TO BE CRYSTALLIZED; WRAPPING SAID BAR IN PLIABLE SHEET MATERIAL NOT WET BY NOR REACTIVE WITH THE MATERIAL OF SAID BAR IN ITS MOLTEN PHASE AND SUPPORTING SAID BAR SO WRAPPED WITHIN A MOLD; PLACING A SINGLE CRYSTAL SEED ADJACENT ONE END OF SAID BAR; WHILE SO SUPPORTED, ESTABLISHING A MOLTEN ZONE AT SAID ONE END OF THE BAR; AND TRAVERSING THE ZONE LONGITUDINALLY ALONG THE BAR. 